Road surfacing material



U d tates Patent ROAD SURFAfIING MATERIAL N Drawing. Application May 6, 1957 Serial No. 657,036

Claims priority, application Great Britain June 21,- 195g Claims. (Cl. 106-284) This invention relates to a method of and materials for surfacingroads, pavements, airfields andthe like (hereinafter and in the claims referred to simply as roads).

Hitherto it has been the practice to utilise for road surfacing a mix of stone, such as whinstone, crushed to the necessary size, and a binder of bitumen, tar or the like. It was common in making roads to apply a base coat say one and a half to three inches deep and then to apply on top a wearing coat or carpet about two inches deep. Commonly the stone in the base coat was of larger size than in the wearing coat. Furthermore, it was known to make a mix of small stone, brown sand, and lake asphalt as binder which was used for an asphalt road surfacing.

. One of the inherent disadvantages of such surfaces has been that with passage of time the bitumen or asphalt binder deteriorates owing to the weathering effects of moisture, rain, sun, freezing and thawing, and also abrasion by road vehicles. Furthermore, the presence of oil, petroleum etc. and exhaust gas deposits from motor vehicles also speeds up the deterioration. As a result the surface begins to break up. The deterioration is progressive, but normally after two to three years substantial deterioration has occurred and break up spreads more quickly.

An object of the invention is to provide a road surface which will resist breaking up for a substantially longer period and which moreover may be less costly to provide.

According to the present invention we provide -a method of forming a road surface, comprising mixing together a binder for example bitumen or tar, and basic open hearth or Bessemer slag, applying the mixture to the road under pressure sothat the binder causes the particles to adhere together, and allowing the surface to weather whereby as the binder deteriorates hydration Y of the silicate particles of the slag develops and secondary adhesion involving chemical binding takes place between the surfaces of the particles thus transforming the whole-into a monolithic structure.

Preferably there is included additional road surfacing 'material for example stone chips, such additionalv material together with the slag ultimately being transformed into a substantially monolithic structure.

Preferably said slag is a basic open hearth slag having a silica content in the form of dicalcic silicate and/or tricalcic silicate of between substantially 10% and 28% by weight there being substantially no free silica or free lime in the slag; alternatively said slag is a basic Bessemer slag having a silica content in the form of dicalcic silicate and/or tricalcic silicate of between substantially 10% and 15% by weight, there being substantially no free silica or free lime in the slag.

The invention further comprises a road surface formed in accordance with the preceding invention and developed intoa substantially monolithic structure by hydration p of the silicate in the slag.

2,875,084 Patented Feb. 24, 1959 lice aggregate. The fines or fine particles of the slag may be equivalent to the sand normally used in asphalt, and such synthetic asphalt material may be used for road surfaces, and as it. weathers and the binder deteriorates hydration occurs as above, again resulting in a monolithic structure.

It is preferred that the surface material comprises simply the aforesaid slag and the binder, but in practice it may often be desired to include aggregate such as stone chips. The-slag will normally be considerably less costly than the stone Various types of bindermay be utilised but it will normally be one which is insoluble in water; it may be mixed-with the slag (and stone if any) when heated, or a binder may be used which is liquid in the cold state and thus does, not require heat for the mixing.

In general terms an asphalt or a synthetic asphalt as referred to herein is of close texture, the particles thereof passing say a three-sixteenths of an inch British standard screen (B. S. 3.); while in a road carpet with an open texture the particles pass a mesh of size greater than three-sixteenths of an inch. Such asphalt or synthetic asphalt is commonly'used for aroad wearing carpet, pavements and the like, while an open texture is'usually used for the base coat. Stone chips of the appropriate size may be used in both cases, or as herein explained, they may be omitted.

Various types of-slag are formed as byproducts-in steel or iron-making processes, for example; metal mixer slag, metal receiverslag, electric furnace slag, cold blast furnace slag, hot blast furnace slag, acid open hearth slag, basic-open hearth slag, acid Bessemer slag, and basic Bessemer slag. 7

Of these slags the only ones suitable for the present invention are the basic open hearth and basic Bessemer slags. Such slags have their content of silica in the form of dicalcic silicate (2CaO-SiO and/or tricalcic silicate (3CaOSiO which provides the secondary chemical bond'aforesaid.

Such slags are examined-and tested and if the silica content is between- 10% -and 28.% by weight in the case of basic openhearth slag, or 10% and 15% in the case of basic Bessemer slag, and if there is substantially no free lime or free silica, it willbe suitable for the present invention. vThat. is to say, both the lime and the silica must be in the. combined form as silicate of lime.

For example, it may often happen in steel making that the lime is added too late in the process, or it may not be properly fiuxed, and thus theremay be residual free lime which would render the slag unsuitable for road surfacing under the invention. Likewise, if 'sand or other free silica is added to the slag ladle this may leave residual free silica, making the slag again unsuitable for the present invention. I

The slag incorporated in the present invention can be regarded as a synthetic high temperature rock of a .uni-

formity in composition and texture which excels that of the commonly used natural stone. Examples of open hearth basic slags suitable for the purpose are those made 0 by the hot metal process using molten pig iron, and also 'tain over seventy percent of'dic'alcic silicate, the slag being extremely solid strong and hard and having a somewhat vitreouslustre, while in another quickly cooled basic open hearth slagsubstantialquantities of tricalcic silicate were present. This silicate is formed in open hearth slags s ereos;

when the amount of lime used is in excess of that required to form the di-silicate. Both types of silicate can therefore exist in the same slag in quantities which are determined by the available lime.

In a slowly cooled basic open hearth slag a substantial quantity of tricalcic ferrite was found to be present resulting from the combination of lime with ferric oxide. It is to be noted that excess of lime in the slag above that necessary to satisfy the chemical aflinities of the dicalcic and tricalcic silicates may be safely fixed by its combination with ferric oxide (Fe O forming tricalcic ferrite (3CaO-Fe O The presence of the last indicates that danger is not to be expected from the existence of free lime which might swell in the presence of moisture and cause blisters on the road surfaceand thus condemn the road. Ferric oxide may be present in basic open hearth slags and thus be available for fixation of excess lime. On the other han'dthe ferric oxide is not available in blast furnace slags and so called falling slags. The presence of t'ricalcic ferrite is thus not essential but'm'ay be an insurance against blistering.

In both basic open hearth and Bessemer slags there is preferably not more than approximately 12% by weight of phosphorus pentoxide. The higher the silica content the better, provided it is in the form of the di or tricalcic silicate; as aforesaid the silica unites with the lime in the slag to form these silicates. Thus dicalcic silicate (2CaO-SiO consists of 34.8% silica and 65.2% lime, so that each 1% silica in the slag is equivalent to ;%%=2.s7% dicaleic silicate It is to be noted, however, that although a high silica content is desired for the road surface, steelmaking considerations normally require reduction of sulphur and phosphorus content in the steel and tend to involve a slag with greater lime dilution and consequently a lower percentage of silica. Slags therefore have varying ranges of silica content.

In the case of basic Bessemer slags the 'same principles apply, but the tendency is for the phosphorus content to be higher and the silica lower because of additional lime required to remove the phosphorus from the steel.

The slag is usually prepared before use by breaking, grinding and sizing. I

As above indicated the presence of the silicates in the material ultimately converts the material into a monolithic block of high durability. As soon as the bituminous or other binder deteriorates, hydration causes the formation of a very thin layer of calcium hydrosilicate on the finest exterior particles of the slag containing di and tricalcic silicate. Because of its jelly-like colloidal nature this surface layer'seals off the exterior of the larger masses and stops further water penetration of the aggregate. Further hydration ceases and the chemical-cementing action of the silicate sets in and forms a chemical bond between the surfaces of the particles thus transforming the aggregate into a monolith.

The initial action of the binder is'to enclose the aggregate grains, but as road carpets are put into service often as little as twenty-four hours after laying, the action of trafiic soon removes the binder coating from the immediate load-bearing surfaces of the topmost grains of the aggregate, and some'of it may be forced or settled down to a lower level; the cementing weathering action to form the secondary chemical bond commences soon on the topmost grain surfaces as indicate'd'above.

This monolithic structure forms as 'a skin at the top part of the road carpet, the part below being protected and retaining the bituminous bond. As and when further deterioration of the bitumen occurs the chemical bond automatically takes .its place and extends the monolithic structure. The time for formation of the chemical bond depends on local climatic conditions and road usage, but by way of illustration, the colloid covering may be noticed 4 in a few weeks, and the monolithic cementing action after a few months.

Road surfaces made in accordance with the invention are unique because the aggregate is double bonded by (a) the adhesiveness of thebituminous coating and (b) the bond formed by the hydration of the di and tricalcic constituents. The road surface has a very low water absorptive capacity and is very resistant to the penetrative effects of freezing and thawing and weather in general, while the texture inherent in the high basicity of the material is such that the-adherence of the binder exceeds that of natural stone. Moreover the high temperature of origin of the slag, which isusually made and matured in a furnace at a temperature 'which exceeds 1600 C., gives absence of cleavage planes anduniformity of cubic fracmm on crushing. This type of fracture greatly assists uniformity of coating, density of packing, and stability of the road carpet. Furthermore the basic slag has a specific g'r'avity whichexceeds that of other common aggregates because of its composition and high temperature formation, and this high density is important for a good road surface. Finally the inherent texture of the road surface provides improved anti-skid properties and additional safety for the road user.

Both bitumen and whinstone, for example, are much more expensive than is the basic slag, and this permits substantial savings in cost especially if it is possible to avoid using the bitumen binder.

The following compositions are given by way of example, percentages being by weight.

(1(a)) A single course road carpet: Percent Binder (cut back bitumen) 7 Crushed basic open hearth slag (three-quarter inchone-eighth inch) 50 Crushed basic open hearth slag (passing No. 200

B. S. S.) 9

Crushed basic open hearth slag (passing one-eighth The binder (bitumen or tar) is heated to 230 F., and the slag is dry and at a maximum temperature of 140 F. and is mixed with the binder.

(2) A non-skid synthetic asphalt for a wearing course:

Percent Binder (bitumen) 6 Crushed basic open hearth slag (passing three-sixteenths inch and retained on one-eighth inch)--- 25 Crushed basic open hearth slag (passing one-eighth inch and retained on No. 52 B. S. S.) 44 Crushed basic open hearth slag (passing 52 B. S. S.

and retained B. S. S.) l5 Crushed basic open hearth slag (passing 100 B. S. S.) 10

inch 20-30 Crushedbasic open hearth slag passing one-eighth Crushed basic open hearth slag (passing three-six with the stone handled along with the slag.

Preferred range- Percent inch and retained on No. 52 B. S. S 25-45 Crushed basic open hearth slag passing No. 52

B. S. S. and retained on 100 B. S. S; 10-20 Crushed basic open hearth slag passing'No. 100

B. S. S 5-15 (3) A fine synthetic asphalt: Binder (bitumen) 9 10 Crushed basic open hearth slag (passing oneeighth inch and retained B. S. S. 52) 50 Crushed basic open hearth slag (passing B. S. S.

52 and retained B. S. S. 100) 25 Crushed basic open hearth slag (passing B. S. S.

In general the broad ranges of the materials envisaged are as follows: Percent 20 Binder (bitumen) 836-936 Crushed basic open hearth slag passing oneeighth inch and retained on No. 52

B. S. S 40-60 Crushed basic open hearth slag passing No. 52

B. S. S. and retained on No. 100 B. S. S 20-30 Crushed basic open hearth slag passing No. 100

B. S. S 10-20 In Examples 2 and 3 the slag is thoroughly dried 9 and fed at (300-400" F.) into a mechanical mixer, with requisite proportions of the binder. The binder is previously heated to not more than 350 F. After thorough mixing the mixture is supplied to the site at not less than 250 F. for spreading.

(4) A stone-filled synthetic asphalt for a wearing course: Percent Binder (bitumen) 6 Whinstone chips (passing three-quarter inch)---" teenths inch and retained by one-eighth inch) 15 Crushed basic open hearth slag (passing one-eighth inch and retained by 52 B. S. S.) 24 Crushed basic open hearth slag (passing 52 B. S. S.

and retained by 100 B. S. S.) 15 Crushed basic open hearth slag (passing 100 B. S. S 10 100 In general the broad ranges of the materials envisaged are as follows: Percent Binder (bitumen) 6-8 Whinstone chips passing three-quarter inch 25-35 Crushed basic open hearth slag. passing threesixteenths inch and retained on one-eighth inch 10-20 Crushed basic open hearth slag passing one-eighth inch and retained on No. 52 B. S. S 20-30 Crushed basic open hearth slag passing No. 52

B. S. S. and retained on No. 100 B. S. S 10-20 Crushed basic open hearth slag passing No. 100

B. S. S- 7-13 Mixing is effected similarly to Examples 2 and 3 but Percent Binder (road tar type B coal tar)........ 4 Whinstone chips (passing one-half inch and retained on one-eighth inch) 45 Whinstone chips (passing No. B. S. S.)....-. 3 Crushed basic open hearth slag (passing threequarter inch and retained on one-half inch) 48 The stone and slag entering the mixer are surface dry and mixed at a maximum temperature of F. The binder is heated to 230 F.

The slag used in each case had a silica content as aforesaid between 10% and 28% by weight, and there was no free lime or silica. Where a basic Bessemer slag is used the proportions and treatment are identical with those above, except that the silica content is of course between 10% and 15% by weight.

We claim:

1. A road surfacing material comprising a water insoluble bituminous binder mixed with crushed slag selected from thegroup consisting of basic open hearth and basic Bessemer, slag.

2. A road surfacing material according to claim 1, in which the slag is crushed, ground and sized.

3. A road surfacing material comprising a water insoluble bituminous binder and an aggregate mixed with crushed basic open hearth slag having substantially no free silica and no free lime but having a silica content in the form of a material selected from the group consisting of dicalcium silicate, tricalcium silicate and mixtures thereof, of between substantially 10% and 28% by weight.

4. A road surfacing material according to claim 3 in which the slag is crushed, ground and sized.

5. A road surfacing material comprising a water insoluble bituminous binder and an aggregate mixed with crushed basic Bessemer slag having substantially no free silica and no free lime but having a silica content in the form of a material selected from the group consisting of dicalcium silicate, tricalcium silicate, and mixtures thereof, of between substantially 10% and 15 by weight.

6. A road surfacing material according to claim 5 in which the slag is crushed, ground and sized.

7. A road surfacing material according to claim 1 wherein the bituminous binder is coal tar.

8. A road surfacing material according to claim 1 in which the slag has substantially no free silica and no free lime.

9. A road surfacing material according to claim 1 wherein the slag is basic open hearth slag having sub stantially no free silica and no free lime but having a silica content in the form of a material selected from the group consisting of dicalcium silicate, tricalcium silicate and mixtures thereof, of between substantially 10% and 28% by weight.

10. A road surfacing material according to claim 1 wherein the slag is basic Bessemer slag having substantially no free silica and no free lime but having a silica content in the form of a material selected from the group consisting of dicalcium silicate, tricalcium silicate and mixtures thereof, of between substantially 10% and 15 by weight.

(References on tollowhig page) r7 References Cited in the file of this patent FOREIGN PATENTS UNITED STATES PATENTS 1 j 2,586 Great Britain of 1871 12 0 Tucker July 2 1 72 18,210 Great Britain of 1894 1,505,880 Fairlie Aug. 19, 1924 26539 Great Britain of 1905 550 475 Hersberger 24, 1951 5 79 G t Britain Kb. 6. 92

521,626 Great Britain May 27, 1940 

1. A ROAD SURFACING MATERIAL COMPRISING A WATER INSOLUBLE BITUMINOUS BINDER MIXED WITH CRUSHED SLAG SELECTED FROM THE GROUP CONSISTING OF BASIC OPEN HEARTH AND BASIC BESSEMER SLAG. 